I don't believe it's silicone, what I did was probe at it with a very small (sharp) precision screw driver and decided it was pretty hard insulation. Also it seemed quite stiff. To determine the gauge, well that was done visually comparing it to a wire in hand which I then measured (real accurate I know). I was more interested in satisfying myself as to what was used. BTW if chassis ground was used, I didn't see it, that would make the wire gauge on the B+ less critical as only half the length of wire is used.